The buoyant force and gravity compete to determine where the
Cartesian Diver goes:

You will need

1) A plastic soft drink bottle full of water and its lid
2) Plasticine
3) A Cup
4) A pen lid. A transparent one works best.

What to do

1) The pen lid will have a hole in it where the pen goes. Stick
some plasticine around the hole, so that if you place it in water it will
float with the hole pointing down and the tip just barely above the surface.
If your pen lid has a hole in the tip, block it off with plasticine. If you
are as equiptment challenged as i was when I filmed the video of this, you
can experiment with hair ties, and rubber bands to weight the cap. Don't block
the hole in the bottom of the pen lid.

2) Use the water in the cup to test if it is floating correctly.

3) Fill the bottle with water, right to the top.

4) Place the Diver in the bottle - make sure he stays right
way up so he doesn't fill with water.

5) Screw the lid on the bottle.

If the diver sinks as you screw the lid on, then it just a little too heavy
- remove a tiny bit of the plasticine from the diver.

What Happens When You Squeeze the Bottle?

Squeeze the bottle and the diver will sink. Release the bottle and the diver
will float.

Be careful: If the bottle is shaken or turned upside-down,
the bubble can escape the diver. You will need to take the diver out, shake
the water out of it and return it to the bottle. You can pour the water into
a bucket and then pour it back into the bottle

How can you turn a full glass upside down and not spill the
water?
Turn a glass of water upside down and the water always falls out. Gravity
pulls the water towards the earth, right?
Is there a way to turn the class upside down without it spilling out?

What You Will Need

1) Large Bowl of Water

2) Food Coloring

3) Small Clear cup

What to Do

1) Make sure you’re doing this somewhere a spill wont
cause a problem – there is a reasonable chance that water will hit the
ground during this experiment.

2) Mix food coloring through the water.

3) Submerge the cup in the bowl, right way up so that it fills
with water.

4) Keeping it fully submerged, turn the glass upside down.

5) Slowly lift the glass, but don’t lift the top of the
glass above the surface of the water.

When you lift the base of the cup above the surface of the water, gravity
tries to pull it back into the bowl.

However, the pressure of the air pushing down on the surface of the water
forces it to remain in the cup.

The atmospheric pressure at the surface of the earth can support a column
of water approximately 10 meters high – above that height, even the
pressure differential between the air and the vacuum that forms above the
column when if drops cannot overcome the weight of the water.

When you lift the glass above the surface, the air can get in
to the top of the glass by easily letting the water fall, so the glass empties.

If you push a pin, or a skewer or some other sharp object into
a balloon, it bursts. Well, actually, I can think of two cases where it won’t
burst:

1) If the balloon isn’t inflated (ok, that’s a bit
silly, but it turns out that it’s to the point). If you poke a hole
in an empty balloon, then there is no explosion, no “bang” and
no little pieces of rubber everywhere. This seems to fit the description of
the balloon not bursting.

2) This is the more surprising case. Take you uninflated balloon
and blow it up, but don’t blow it up too far. Once you have got a few
breaths into it, pinch shut the end and examine the balloon. You’ll
notice that around the opening and around another point roughly straight across
the balloon from there, the rubber is unstretched.

You can tell it’s unstretched, because unlike the rest of the balloon,
it is not translucent (i.e. it doesn’t let any light through), as rubber
stretches, it gets thinner (just like if you stretch a piece of gum) and when
it gets thin enough, it starts letting light through. If you poke a needle
or some other sharp object into these dark areas, the balloon will not burst.
In my experience, I’ve found that barbeque type skewers work very well,
with the added advantage that you can usually work them right through the
balloon and out the other dark patch. The balloon does have a hole in tit
now, and if you listen closely, you’ll be able to hear the air escaping
it, especially if you pull the sharp object out again, but in many ways it
keeps the properties of a balloon. The most important property for us is “burstability”
because the people you show this too are going to want proof that it’s
not a trick balloon: so while the balloon is still skewered, poke it with
another skewer, on the thin side. It should burst nicely.

Why doesn’t this balloon burst? Why do balloons burst
anyway? Balloons are made of rubber, which is an elastic material, meaning,
if you stretch it, it pulls back.

In order to make a hole in the balloon, you need to push the skewer into the
side of the balloon until the rubber in front of the point is so stretched
that it breaks. Then two things can happen – either the balloon bursts,
or it doesn’t. If the rubber you’re poking through is generally
unstretched (like the end of an inflated balloon or an uninflated one), then
the stretching due to the skewer is in a sense “local”, that is,
only the rubber very close to the point is stretched and breaks., the rest
of the rubber remains outstretched and holds together.

Around the hole the skewer made is a number of little cracks
and tears in rubber. If the rubber is slack, these don’t spread and
the balloon stays together. On the other hand, if the rubber is stretched,
then it pulls on these cracks and tears and makes them larger and larger.
Some of these tears very quickly become large enough that the balloon falls
to pieces. This is when you burst a balloon by making a small hole in it,
you still often end up with the balloon looking like it was torn to pieces.

Another way to prevent these tears catastrophically increasing
is to reinforce the balloon in some other way. For example, if you put a strip
of sticky tape on the balloon and carefully pierce the balloon through the
tape, the tape should hold the tears together and kept he balloon in one piece.

But where does the bang come from?

The balloon is full of air at high pressure, held in by the
balloon. Once the balloon is gone, there is nothing holding in the air, so
it tries to spread out and equalize the pressure everywhere. This cannot happen
instantaneously, so a “wave” of high pressure air spreads out
from the balloon. Waves of high pressure air are exactly what sound is, so
when this high pressure hits your ears, it “makes the bang”.

High pressure air balloon rockets and momentum.

It is the pressure of the air inside that holds the rubber sides
of a balloon out, so it’s fun to play with. Balloons also give us another
way to look at our last idea – air will try to move so that it equalizes
any variation in pressure – when you blow up a balloon, but don’t
tie off the end, the air will rush out so that the pressure inside and outside
will be equal.
When the air rushes out the end of the balloon, it makes the balloon shoot
forward – another interesting effect called conservation of momentum
– one of the most fundamental physics laws in the universe. Momentum
is measure of how much “motion” an object possesses. In a sense,
it is momentum that determines how hard a thrown ball hits your hand. A ball
that is moving fast will hit harder than a ball moving slowly. A heavy medicine
ball will hit harder than a baseball (going the same speed). Physicists have
conducted hundreds of thousands of these collision experiments and determined
that moment (which is equal to mass times velocity) is never created nor destroyed.
When you catch a ball, its momentum is transferred to your hand – if
you’ve caught a ball moving fast enough, you’ll have noticed that
it pushes your hand backwards, which pushes your body backwards.

You don’t go flying back as fast as the ball for two reasons:

1) you’re much heavier than the ball, so the velocity it gives you is
much smaller than the velocity it had, and

2) the friction between you and the ground transfers the momentum into the
earth, which is so heavy that the momentum going into it is unnoticeable (and
pretty much cancel out the momentum taken from it by the person who through
the ball in the first place)

If you lower the pressure in your mouth, the air pushes the top of the water
up the straw!

When you suck water through a straw, what happens?

First we need to talk about pressure. The gas molecules that make up air wiz
around and bump into things. Like a ball bouncing against a wall (or your
hand) when the gas bumps into things, it pushes against them. This pushing
is exactly what we mean by pressure – and even if you think you’ve
never experienced, I’ll bet if we think about it a little, I can convince
you of this. Let’s try a little experiment

1) Take a deep breath

2) Breath out slowly and steadily through your mouth

3) Before you’ve finished breathing close your mouth –
but don’t let the air out through your nose either, but continue blowing
out against the inside of your cheeks

Feel something pushing your cheeks out and trying to part your lips? That
is air pressure. When you’ve got your mouth open, the air on the in
and out sides of yourcheeks will be at the same pressure – so the air
outside will push in with the same force as the air inside pushes out –
so you won’t feel anything, but when your ribs squeeze the air out of
your lungs, there’s more air jammed into the same space within your
mouth – so it’s like having two or three or twenty (depending
on how strong your lungs are) times as many balls bouncing against the inside
walls – the pushing out is stronger than the pushing in, which stretches
out your cheeks.

Another example of this is a balloon – the pressure of the air inside
holds the rubber sides of a balloon out, so it’s fun to play with. Balloons
also let us into another important idea – air will try to move so that
it equalizes any variation in pressure – when you blow up a balloon,
but don’t tie off the end, the air will rush out so that the pressure
inside and outside will be equal. When the air rushes out the end of the balloon,
it makes the balloon shoot forward – another interesting effect called
conservation of momentum – one of the most fundamental physics laws
in the universe. But it’s the tendency to equalize pressure variations
that helps us drink through straws.

Now let’s try the reverse of our last experiment –
keeping your mouth closed, suck your cheeks in. Your lungs are expanding,
reducing the amount of air in your mouth – now it is the air outside
that wins in the contest to push you r cheeks. But as you suck your cheeks
in, air will sneak in to your mouth, pushing your lips apart to do so. Air
is that determined to bring back the equality of pressure, that it will move
parts of your body!

So how is this related to drinking? When you suck through a
straw, you’re doing exactly the same thing – but instead of pushing
your cheeks in, the air outside has found a better way to get into your mouth
– by pushing down on the top of your drink so that it shoots up the
straw into your mouth. In order to get into your mouth, the air is willing
to push all the drink in your glass all the way up to your mouth!

2) Place the other end of one of the straws in your drink, but leave the second
straw in the air

3) Try to drink!

You’ll notice that no matter how hard you try to suck up the drink,
all you end up getting is a mouthful of air from the second straw. Although
nature is determined to equalize the pressures, it’s lazy – pushing
air up through the free straw is much easier than pushing fluid, so only air
flows into your mouth.

4) now, if you were lucky, or especially clever, at the last
step, you might have succeeded at getting a drink – if you use part
of your lip, or your tongue to seal the end of the free straw tightly, air
will no longer be able to go up that straw, and you’ll be able to drink

5) Challenge others to “two straw drinking races” – but
don’t tell them the secret. Use this to impress your friends, family
and coworkers

A subtler version of this trick is to make a very tiny hole
(or have an adult make the hole if you’re not allowed to use knives)
in the side of a drinking straw about half way up. Now, if you try to drink
with this straw (with the new hole outside your mouth and above the top of
the drink) , you’ll find that you just get air – now try it with
your finger over the hole

The pushing force of air is called air pressure. The closer you are to Earth,
the greater the air pressure. The farther away from Earth (in other words
the higher your altitude), the less the air pressure. And remember, pressure
is coming from all around us.

What to do:

1) Take the coffee can and punch 3 small holes in the bottom. Also punch one
hold in the plastic lid.

2) Now fill the coffee about 1/2 full of water and put the lid on.

3) Place your hand over the hole and press down on the lid. Notice how the
water streams out of the holes on the bottom due to the pressure you are exerting
on the lid.

4) Now slowly stop applying pressure to the lid. Notice how the stream of
water stops. You can stop and start the flow of water simply by removing you
finger from the hole. (Now would be a good time to hand the can to one of
your parents...)

5) When you filled the can only half full, you left some space empty. This
space actually was not empty - it was filled with air. Pressure on the lid
exerted pressure on this air which in turn exerted pressure on the water forcing
it out of the can.
When you stop pressing on the lid, and leave your finger over the hole, the
pressure of the air outside the can holds the water up from the bottom.

Many years ago WHAM-O sold a plastic air-puff gun. The puffs of air could
fly across a room and knock over cardboard targets.

It turns out that this gun used ring-vortices, or "invisible
smoke rings" as its ammunition. Also turns out that smoke-ring guns are
extremely easy to make.

What you need:

1) A soup can

2) A piece of cardboard

3) A balloon

What to do:

1) Take a soup can, cut out the top and bottom, tape a piece
of cardboard over one end, and cut a 1" hole in the center of the cardboard.

2) snip a balloon in half and stretch it across the other end.

3) When you gently whack the covered end of your vortex launcher,
a transparent ring of spinning air will shoot out of the hole. Aim the device
at your face or arm, and you'll feel the puff of air when it hits your skin.

4) The vortex rings can be made visible with a bit of smoke.
I use stick incense, and just shove the end of the stick into the hole for
awhile (don't set the cardboard on fire!!)

5) Tap the bottom gently, and slowly spinning smoke rings will
be launched. Tap it hard, and the smoke rings will zoom so fast that you'll
only see a grey blur. Tap it too hard and you generate air turbulence but
no smoke rings.

To see the details of the smoke rings it helps to have bright lights and a
dark background. Work in a darkened room while placing your device between
you and a bright table lamp. The light should shine towards you, through the
smoke, but position things so you observe the smoke against a darkened wall.
Smoke rings are similar to tornadoes, but the ends of the tornado is curved
around so its ends are joined into a circle.
Try shooting slow rings then immediately shoot faster ones. The faster ones
will catch up to the slower ones and move through them (the slower ones open
wider to allow the fast ones to pass.)
Rather than using smoke, you could instead use scent. Any fumes in the can
will end up inside the air in the smoke ring. Try putting perfume in the can.
When you launch your ring vortices, they will be invisible. But if you target
a distant nose, your victim will know when they've been hit.

First we'll show that there is air pressure pushing on us, from
every direction while we're on this Earth.

You will need:

1) Newspaper,

2) ACE' hardware yardstick (1/8"
thick)

3) a flat table

What to do:

1) Place a thin yardstick on a flat table with a little less
than half of it hanging off of the edge of the table.

2) Place a sheet of newspaper over the yardstick flat against
the table (have as little air as possible under the paper) so that the fold
line of the newspaper is at the yardstick.

3) Quickly strike the end of the yardstick hanging off the edge
of the table. If you strike it quick enough, the yardstick will break near
the table edge.

The Earth is covered in a layer of air that is nearly 80 miles thick and at
sea level (the bottom) exerts or 'pushes' almost 15 pounds of pressure per
square inch. That means that a full sheet of newspaper laid out flat has nearly
9,300 pounds of air above it. When you break the yardstick above, you are
able to break it because of that 'heavy' air pushing down on the paper while
you quickly strike the yardstick. Initially, the table is pushing back on
the paper, and if you move the yardstick quick enough, other air around the
edges of the paper can't get under the paper fast enough, so you are trying
to lift that 9,300 pounds with the yardstick! Some air gets under the paper,
but not enough, so the yardstick breaks.

Second Experiment:

Now, we're going to make a balloon 'rocket' that shoots along
a kite string.

You will need:

1) Kite string

2) plastic straws

3) balloons

4) cellophane or masking tape

What to do:

1) Cut a plastic straw in half and tie a length of string (at
least 20 feet long is more fun) between two chairs or something.

2) Before you tie the second knot in the string, slip the straw
on to the string. Try to get the string fairly tight (the two chairs work
well because you can pull the chairs apart to get the string tight).

3) Blow up a balloon, but don't tie off the end and tape it
to the straw so that it resembles the drawing below.

4) Let go of the balloon and the 'rocket' should shoot along
the string (very quickly) towards the other chair. Try a different kind of
balloon!